Almost irrelevant at DC, somewhat tricky for audio, and bloody essential
for RF work.
Anyway you like.
As large as will fit, is not a bad way to start. You need a lot for high
currents as well, and it also helps heat to get away.
Anyway you like the look of, but I'm assuming (prelude to disaster)
you mean a linear PSU. Switchers can be a little tricky in places.
Not in the slightest.
Anyway you like, it's maybe less confusing to lay them out much like
the schematic, but it looks better if they are laid in some pattern.
Depends on two things, the current on the 5volt side determines the
width of the tracks. For mininal temp rise on normal PCB material you
should use the following rough figures:
1 Amp = 25 thou
4 Amp = 75 thou
8 Amp = 250 thou
If you are going to use the supply voltage (110 or 230 vac) on the
primary side, you can use 25 thou tracks, but be careful to allow a
GOOD safety spacing between these and the 5 volt side or any
metalwork. I would recommend 0.5 inch. You are permitted to use less,
but it sounds like you are a raw beginner. Take care!